High resolution atom probe

ABSTRACT

A three dimensional atom probe comprising a sharp specimen ( 10 ) coupled to a mounting means ( 12 ) where emission of charged particles is caused by application of a potential to the specimen tip ( 10 ) such that charged particles are influenced by filtering electrodes ( 206, 204 ) before impingement on a detection screen ( 202 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This appiication is a U.S. national phase application ofPCT/US2004/016590, filed May 26, 2004, which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/476,348, entitled “HIGHRESOLUTION ATOM PROBE,” filed Jun. 6, 2003, which is incorporated hereinin its entirety by reference.

FIELD OF THE INVENTION

This document concerns an invention relating generally to the field ofatom probe microscopy.

BACKGROUND OF THE INVENTION

The three-dimensional atom probe (3DAP), also known as aposition-sensitive atom probe (POSAP), is a device which allowsspecimens to be analyzed on an atomic level. Typical atom probes operateby (usually positively) ionizing and extracting atoms from a specimen'ssurface. FIG. 1 presents a schematic view of an exemplary atom probe ofa more recent type, wherein the locations and identities of thecomponent atoms of a specimen 10 are determined by situating thespecimen 10 opposite a position-sensitive detector 100 (generally amicrochannel plate and delay line anode). A local electrode 102 is thensituated between the specimen 10 and detector 100. The specimen 10 isgenerally charged to some datum potential Vs (generally a positivevoltage between 500 and 20,000 V), and the local electrode 102 is heldto some attractive potential Vle. Where the datum potential Vs ispositive, the local electrode 102 is often set to ground (0 V). Thedetector 100 is also charged to a potential Vd which is attractive withrespect to Vs. As a result, the atoms of the specimen 10 are attractedtowards the local electrode 102 and detector 100 in accordance withtheir proximity from the local electrode 100 and detector 100 (i.e.,atoms of the specimen 10 which are closer to the local electrode 100 aremore strongly attracted). However, the magnitude of Vs−Vle—i.e., theattractive force exerted between the local electrode 102 and thespecimen 10—is held at some fraction of the value necessary to ionizeatoms of the specimen 10.

When it is then desired to ionize atoms, an additional attractivepotential—an “over-voltage” Vo—is then momentarily applied to the localelectrode 102, usually in brief pulses, so that the total appliedpotential Vs−(Vle+Vo) will induce atoms to “evaporate” from the specimen10, ideally with a single atom (ion) of the specimen 10 leaving thespecimen 10 each time an over-voltage pulse is applied. Additionally oralternatively, momentary heating of the specimen 10 (as with a laser)can be used to induce ion evaporation. The evaporated ions areaccelerated towards the local electrode 102 to pass through an aperturedefined therein, and then impinge upon the detector 100. Under ordinarytest conditions, the evaporated ions from the specimen 10 project ontothe detector 100 at positions correlated with their original locationson the specimen 10, and thus the detector 100 provides data regardingthe original position of the ions on the specimen 10. Additionally, iontimes of flight (as measured between application of the over-voltagepulse Vo and detector impingement) provide information regarding ionmasses, and thus their identities. Thus, repeated overvoltage pulsingallows a three-dimensional map of the locations and identities of theatoms of the specimen 10 to be constructed. Further general informationmay be found, for example, in U.S. Pat. No. 5,440,124; U.S. Pat. No.5,061,850; International Publication WO 99/14793; and Kelly et al.,Ultramicroscopy 62:29–42 (1996).

One performance limitation of atom probes is their ability todistinguish between ions of nearly similar masses. This property, knownas mass resolution, limits the ability to accurately identify ions fromthe specimen, and leads to uncertainty or errors in the compositionalanalysis provided by atom probes. Mass resolution limitations are aconsequence of the probabilistic nature of ionization: the precisemoment at which ionization occurs during an overvoltage pulse Vo canvary slightly between pulses, and thus there are limitations inprecisely determining the time of evaporation. Additionally, owing topractical limitations and expense, pulsing systems for applying theovervoltage pulse Vo tend to apply a pulse wherein the amplitude of Vois not constant over the duration of the pulse, and thus the exactescape potential (and thus velocity and time of flight) of an evaporatedion will vary. While these limitations are expected to diminish as theavailable pulsing electronics grow in quality (and decrease in expense),they nonetheless pose difficulties given the state of the art as of theyear 2003. Several strategies have been employed in atom probes toincrease mass resolution, and one strategy (noted in some of theforegoing references) is to situate an intermediate electrode 104closely adjacent to the local electrode 102, and between the localelectrode 102 and the detector 100, and to hold it at some constantattractive potential Vi whereby the velocity of evaporated ions will (atleast to some extent) be decoupled from the overvoltage pulse Vo used toinduce evaporation. One strategy is to hold Vi at the same potential asVle, thereby decelerating ions by the amount equivalent to Vo from whichthe velocity variation originates. Alternatively, an accelerating (moreattractive) potential can be applied to Vi, increasing the overallvelocity of the ions so that the variation in Vo becomes relativelysmaller.

Another limitation of atom probes relates to the time and expense oftesting, particularly in the time and expense of preparing specimens foranalysis. Initially, the specimens being analyzed must generally becarefully prepared by removing portions of the specimen around the areaof interest for study, so that the area of interest is at the tip of aneedle-shaped specimen (typically less than 100 nm in diameter). Theneedle shape creates the large electric field conducive for ionization,allowing the atom probe to operate over a more convenient voltage range(and/or allowing use of less complex thermal pulsing systems).Additionally, since the ions from the needle project onto the detector(and generally “spread” onto the detector in accordance with theirrelative positions along the axis of the specimen and local electrode),the needle shape assists in attaining atomic-scale resolution inposition data for detected ions. However, preparation ofspecimens—forming their areas of interest at the tips of needles—can betime-consuming and expensive, and can also be difficult where specimensare brittle or otherwise difficult to shape (as is often the case withsemiconductor wafer-derived specimens).

Additionally, since the atom probe must be located in a vacuum chamber(and the specimen must be cryogenically cooled) for optimal operation,specimen processing can be slowed by the need to load and purge thechamber, and to achieve the desired degree of specimen cooling, beforetesting each specimen. The “warm-up” time between specimens can bereduced by situating multiple specimens within the chamber at the sametime (or by forming multiple microtips in a specimen), and thenlaterally repositioning the specimen with respect to the local electrode(or vice versa) so that several microtips can be analyzed in sequencewithout the need for intermediate load/purge/cool steps. However, thereis still room for improvement in the speed of specimen throughput.

To compound the foregoing problems, specimens are often tested only tofind that the collected data is incomplete, e.g., the data does notfully represent the regions of the specimen of particular interest. Asan example, the atom probe may not have been run for long enough thatdata is collected from the desired depth within the specimen. Data mayalso be incomplete because a desired feature may rest partially orwholly outside of the field of view of the atom probe, because the ionsfrom the feature had flight paths which did not impinge upon thedetector. It is also possible that data from the desired feature may notbe collected at the desired magnification: the detector has a limit tohow accurately the location of ion impingement may be measured, and thusinsufficient spread in ion flight paths may tax the sensitivity of thedetector, resulting in “coarse” positional data. To illustrate some ofthese shortcomings, referring to FIG. 1, the detector 100 is spaced fromthe local electrode 102 at such a distance that acceptabletime-of-flight readings can be made (i.e., so that the desired degree ofmass resolution is obtained). Since there are practical limitations onthe size of the detector 100 (with larger ones generally being on theorder of about 100 mm in diameter as of the year 2003), the detector 100is generally sized such that it only rests within a portion B of thecone of evaporated ions flying from the specimen 10 (this cone of ionsbeing designated by the reference character A). Since the detector 100essentially captures a projection of the specimen 10, the portion of theflight cone A intersecting the detector 100 (i.e., flight cone B)defines the field of view captured by the detector 100: as the detector100 receives more of the flight cone A, the detector 100 will image agreater amount of the specimen 10. If the detector 100 is thenpositioned at 100A, more distantly from the local electrode 102, itintersects even less of the flight cone A—it receives flight cone C, asubset of the ions of flight cone A—and the field of view is decreased.However, situating the detector at 100A will yield greatermagnification, since the ions, when reaching 100A, have greater spread.Additionally, there is some gain in mass resolution (and thus ionidentification) owing to the slightly longer time of flight. The moredistant detector 100A will also provide a greater depth of analysiswithin the specimen 10, assuming that a set number of ions will becollected (since detection of some set number of atoms, e.g., 10⁶ atoms,from a smaller area on the specimen 10 necessarily requires collectionmore deeply within the specimen 10 if the requested number of atoms areto be obtained). The various tradeoffs involved with varying the spacingbetween the detector 100 and specimen 10 may be summarized as follows:

Sampled Mass Distance FOV Magnification Depth Resolution IncreasesDecreases Increases Increases Increases Decreases Increases DecreasesDecreases Decreases

The end result of the foregoing problems is that an experimenter mayundergo the time-consuming steps of specimen preparation, atom probewarm-up, and data collection only to find that the data collected haslittle value: the desired feature is not within the field of view, orhas insufficient magnification, or is not sampled to the desired depth,etc. This is particularly problematic where the specimen(s) are rare,expensive, or one-of-a-kind: there may not be a second chance to obtainthe desired data.

In view of the foregoing issues, it would be useful to have methods andarrangements available which more readily allow the accurate collectionof desired data from atom probe specimens with little or no increase inthe burdens of specimen preparation, probe warm-up, and datacollection/analysis.

SUMMARY OF THE INVENTION

The invention involves an atom probe which is intended to at leastpartially solve the aforementioned problems. To give the reader a basicunderstanding of some of the advantageous features of the invention,following is a brief summary of preferred versions of the atom probe. Asthis is merely a summary, it should be understood that more detailsregarding the preferred versions may be found in the DetailedDescription set forth elsewhere in this document. The claims set forthat the end of this document then define the various versions of theinvention in which exclusive rights are secured.

In preferred versions of the invention, an atom probe includes aspecimen mount whereupon a specimen to be analyzed may be situated,wherein the specimen mount is charged to a datum potential. A detectoris spaced from the specimen mount, wherein the detector bears anattractive potential with respect to the datum potential whereby anyions from a specimen on the specimen mount are attracted toward thedetector. A local electrode is then situated between the specimen mountand detector, with the local electrode also bearing an attractivepotential with respect to the datum potential whereby any ions from aspecimen on the specimen mount are attracted toward the local electrode.Preferably, at least one of the specimen mount and the detector aremovable to adjust the distance between the specimen mount and thedetector, whereby the field of view, magnification, and effective sampledepth of imaged specimens may be adjusted (and the time of flight of thespecimen's ions may be adjusted, thereby adjusting the mass resolutionat which these ions may be discerned).

An intermediate electrode is then situated between the local electrodeand the detector along the ion flight path between the specimen mountand the detector. This intermediate electrode may serve as a focusingelectrode which assists in adjusting field of view and magnification,and/or as a filtering electrode which helps eliminate spurious ions andthereby assists in image data quality. When the intermediate electrodeserves as a focusing electrode (as depicted by the focusing electrode206 in FIG. 2 and the focusing electrode 412 in FIG. 4), its potentialis adjusted about the potential of the local electrode (preferably to apotential between that of the local electrode and the detector), wherebythe flight path of ions traveling adjacent the first intermediateelectrode and between the local electrode and the detector may beadjusted; for example, with reference to FIG. 2, the flight cone may beadjusted from A to the narrower flight cone B, or to the wider flightcone C). As exemplified by the focusing intermediate electrode 412 inFIG. 4, the focusing electrode is preferably also repositionable alongthe ion flight path between the specimen mount and the detector, and itpreferably has a tubular configuration with an interior length orientedalong the ion flight path between the specimen mount and the detector,whereby ions traveling from any specimen on the specimen mount to thedetector travel through the interior length of the intermediateelectrode.

When the intermediate electrode serves as a filtering electrode (asexemplified by the filtering electrode 306 in FIG. 3 and the filteringelectrode 408 in FIG. 4), it is preferably charged to a filteringpotential which is between the datum potential and the potential of thelocal electrode, but closer to the datum potential than to the potentialof the local electrode (i.e., it has a filtering potential close to thepotential of the specimen). This filtering potential may beintermittently applied to the filtering intermediate electrode, as bytiming it in accordance with the overvoltage pulses applied to the localelectrode. As depicted by the filtering electrode 408 in FIG. 4, it isalso useful to have the filtering electrode repositionable between thespecimen mount and the detector, since both its charge and location willhave an effect on filtration of unwanted ions. It can also be useful toprovide radiating members (depicted at 410 in FIG. 4) which extendacross the interior of the filtering intermediate electrode, e.g.,members formed in a grid/mesh (or other configuration having a largeamount of free space for ion flight), so that the filtering electricfield emitted by the filtering electrode remains relatively uniformacross its aperture.

A particularly preferred arrangement is as depicted in FIG. 4, whereinat least two intermediate electrodes are provided between the localelectrode and the detector, wherein at least one serves as a filteringelectrode and at least one serves as a focusing electrode. Asexemplified in FIG. 4, a focusing intermediate electrode 412 may beprovided between a filtering intermediate electrode 408 and the detector404, wherein the focusing intermediate electrode 412 is charged to apotential between that of the filtering electrode 408 and the detector404. The intermediate electrodes may be formed in such a manner that onemay be received within the interior of the other, and by making eitheror both of the electrodes movable along the flight path, the relativepositions of the electrodes can be readily adapted to provide a varietyof focusing and filtering effects (and if desired, the electrodes can beinterfit and can cooperate to effectively serve as a single electrode).

Further advantages, features, and objects of the invention will beapparent from the following detailed description of the invention inconjunction with the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional atom probe shown inconjunction with an analysis specimen 10, and with the detector of theatom probe illustrated at different locations 100 and 100A.

FIG. 2 is a schematic diagram of an exemplary atom probe 200implementing preferred features of the invention, wherein a tubularintermediate electrode 206 and a repositionable detector 202 are used tomodify ion flight paths between flight cones A, B, and C (and thusmodify the atom probe's field of view, magnification, sample depth,etc.).

FIG. 3 is a schematic diagram of another exemplary atom probe 300wherein the intermediate electrode 306 serves as a filteringintermediate electrode for reducing spurious detector readings.

FIG. 4 is a schematic diagram of another exemplary atom probe 400incorporating both a filtering intermediate electrode 408 and a flightpath modification intermediate electrode 412 for adjusting field of view(and thus magnification, sample depth, etc.).

DETAILED DESCRIPTION OF PREFERRED VERSIONS OF THE INVENTION

Referring to FIG. 2, an atom probe exemplifying certain preferredfeatures of the invention is designated generally by the referencenumeral 200. A specimen 10 is situated on a specimen mount 12, which ischarged to a datum potential Vs. A detector 202 is spaced from thespecimen 10, and as in prior arrangements, an attractive potentialgradient is maintained between the specimen 10, a local electrode 204,and the detector 202: surface atoms on the specimen 10 are ionized bythe electric field at the specimen surface as a result of themomentarily applied potential (equal to Vs−(Vle+Vo)), and acceleratedtowards the local electrode 204 and the detector 202.

The detector 202 is situated on a positioning stage 204 which allows thedetector 202 to be repositioned at least along the axis of the flightcone A wherein evaporated ions travel from the specimen 10 whenovervoltage pulses Vo are applied to the local electrode 204. Such apositioning stage 204 may take the form of virtually any type ofactuator capable of operating within the chamber of the atom probe 200(or any form of actuator capable of repositioning the detector 202 fromoutside the chamber via a connecting linkage). Ideally, the positioningstage allows variation of the distance between the specimen and detectorby at least 30%, and it is particularly preferred that thespecimen-detector distance be variable to allow a flight path between40–150 mm.

An intermediate electrode 206, preferably having a tubular form with anelongated interior passage oriented concentrically along the axis of theflight cone A, is then situated between the local electrode 204 and thedetector 202. The intermediate electrode 206 also bears an attractivepotential Vi with respect to the specimen 10. If the potential Vi of theintermediate electrode 206 is set equal to the potential of the localelectrode Vle, the ions within the flight cone will experience a modestdeceleration (corresponding to Vo) when traveling between the localelectrode 204 and the intermediate electrode 206, and the flight cone Ashould be largely unaltered. However, with appropriate tailoring of thepotential Vi applied to the intermediate electrode 206—as by varying itsignificantly above or below the potential Vle of the local electrode204—the shape of the flight cone A may be altered to modify theeffective field of view captured by the detector 202. If the attractivepotential of the intermediate electrode 206 is increased by somepotential Vx, ions within the flight cone A will be accelerated as theytravel between the local and intermediate electrodes 204 and 206,resulting in an effective narrowing of the flight cone (exemplified byflight cone B) as the forward velocity component of the ions (theirvelocity towards the detector 292) is increased in comparison to theirlateral/radial component of velocity (their velocity outwardly from theaxis of the flight cone A). This results in an effective increase in thefield of view captured by the detector 202 (assuming its positionremains constant), with a corresponding reduction in magnification, andin the depth/volume of the specimen 10 being sampled (assuming that aconstant number of ions/atoms are collected). Conversely, if theattractive force is decreased by some potential Vx (i.e., if Vi ispositive relative to Vle), the evaporated ions will be decelerated, andthe angle of the flight cone A will increase as the forward velocitycomponent of the ions is decreased in comparison to their lateral/radialcomponent of velocity (resulting in a flight cone exemplified by flightcone C). As the flight cone spreads, so does the projection of thespecimen 10 on the detector 202, and thus the field of view is decreased(assuming that the detector 202 is maintained in the same position).

The foregoing arrangement therefore allows modification in the field ofview captured by the detector 202 (and thus in magnification andsampling depth). Further ability to modify field of view is allowed byadditionally providing the detector 202 on the positioning stage 204,since decreasing the distance between the detector 202 and the localelectrode 204 will increase field of view (and conversely, more distantspacing decreases field of view). The arrangement therefore allows thedetector 202 to be distanced to such a degree that time of flight (andthus mass resolution) are acceptable, with such distancing providing thedesired field of view in conjunction with the action of the intermediateelectrode 206.

Modification of the flight cone from the typical (unmodified) conicalshape A to a modified shape (such as B or C) will require additionalanalysis of the data generated by the detector 202 in order to properlycorrelate ion impingement with its location and identity on the specimen10: whereas the flight cone A results in a fairly straightforwardprojection of the specimen 10 onto the detector 100, the modified flightcones B and C would, if interpreted in the same way, provide a distortedinterpretation of the specimen 10. Stated differently, when the flightcone is modified, the ion impingements on the detector 202 requirecompensation for the curvature in their paths if they are to be properlycorrelated with their presence on the specimen 10. Data correction maybe affected by applying one (and preferably a combination) of thefollowing methodologies.

First, principles of electrostatics may be used to numerically model theflight path modifications introduced by the intermediate electrode 206,and allow compensation for any changes made to ion flight times andpaths.

Second, corrections may also be derived by statistical analysis ofcollected ion impingement data. Consider, for example, that when theflight cone is spread (i.e., off-axis ions will be pulled furtheroff-axis), the density of ion impingements of ion impingements in theradial direction, outwardly from the center of the flight cone, willgenerally be reduced from the case of an unspread flight cone.Similarly, when the flight cone is compressed by an accelerating Vi, thedensity of impingements will be greater towards the edges of the cone ascompared to an unspread flight cone. Thus, statistical analysis of theradial variation and density of impingements can assist in providing aspread function which compensates for curvature in flight paths.

Finally, empirical testing can also provide correction functions. Bytesting specimens with well defined/well known structures (e.g.,tungsten crystals), and correlating the distorted image captured by thedetector 202 with a modified flight cone B or C to the standard imagethat would otherwise be collected by a conventional flight cone A,correction functions may be derived to compensate for flight pathmodifications.

The geometry of the tubular intermediate electrode 206 is an importantfactor in effecting modification in field of view, and an appropriateshape which will effect the desired spread may be determined by computermodeling and electrostatic analysis. If the intermediate electrode 206simply takes the form of an apertured plate (or apertured plate-likestructure), as in U.S. Pat. No. 5,440,124 and International PublicationWO 99/14793, the change in spread of the flight cone will be minor ornegligible. A basic version of the intermediate electrode 206 involves afrustoconical tube. It is also generally useful to situate theintermediate electrode 206 as close as possible to the local electrode204 (with limitations in spacing being largely dependent on the need toelectrically isolate the local electrode 204 and the intermediateelectrode 206).

FIG. 3 then illustrates another exemplary atom probe, which is generallydepicted by the reference numeral 300. This version 300 corresponds toconventional arrangements (such as that shown in FIG. 1) in severalrespects. A specimen 10 is provided on a specimen mount 12, whichcharges the specimen 10 to some datum potential Vs. A local electrode302 is charged to some attractive potential Vle (i.e., more negativethan a positive Vs), and an overvoltage Vo is then periodically appliedto induce ion evaporation from the specimen 10. Evaporated ions impingeon the detector 304 to generate data whereby an image of the specimen 10may be reconstructed, with time of flight data providing information onion/atom identity.

An intermediate electrode 306 is also provided, similarly toarrangements such as those in International Publication WO 99/14793 andU.S. Pat. No. 5,440,124. As noted in WO 99/14793 (e.g., at page 7, lines4–19), such an intermediate electrode 306 may be set at an attractivepotential (e.g., Vi=Vle) such that the velocity of ions emitted from thespecimen 10 is largely decoupled from the overvoltage pulse Vo. Thisreduces the spread of ion kinetic energies associated with thetime-varying amplitude inherent in the overvoltage pulse Vo.

If operated in the standard arrangement, the atom probe 300 results in aflight cone A incident on the detector 304. However, the intermediateelectrode 306 may also be used as a filtering electrode to reducedetection of spurious ions from the specimen 10. Spurious ion emissioncan occur for several reasons. As one example, ion evaporation from thespecimen 10 may occasionally occur in the absence of an overvoltagepulse Vo. As another example, tramp gas—gas remaining in the atom probechamber after purging—commonly impinges upon the specimen 10, and isthen ionized to eject towards the detector 304. These events can causeseemingly erroneous detector readings because the particle strikes onthe detector 304 may not be coupled to the timing of overvoltage pulsesVo at the local electrode 302, and thus their true time of flight isuncertain. The impingement of such spurious ions often results in acontinuous “noise floor” in the detector data 304 which may obscure thepresence of low-concentration species in the specimen 10.

To reduce detection of spurious ions from the specimen 10, theintermediate electrode 306 may be charged such that 0≦Vi−Vs<Vle−Vs,i.e., the intermediate electrode 306 either has a less attractivepotential than the local electrode 302 or has a repelling potential. Aparticularly preferred arrangement is to set the potential Vi of theintermediate electrode 306 at (or about) the specimen potential Vs.Thus, an ion properly evaporated during the application of theovervoltage Vo will have a potential equal to |Vs|+k|Vo| (where k isbetween 0 and 1, depending on the geometry of the local electrode 302)after leaving the local electrode 302. This ion will be decelerated bythe intermediate electrode 306, slowing the ion by an amountcorresponding to the potential |Vs|, but will still retain a potentialof k|Vo|, allowing it to continue on to the detector 304. Spurious ionsdo not possess the potential associated with the overvoltage Vo and soleave the specimen 10 with a potential equal to |Vs|. As such ionsapproach the intermediate electrode 306, they are slowed by thepotential |Vo| and will not be able to reach the detector 304.

Turning then to FIG. 4, another exemplary atom probe 400 is shown whichin essence combines features of the foregoing versions 200 and 300. Asin prior arrangements, the specimen 10 is provided on a specimen mount12 which charges the specimen 10 to some datum voltage Vs. A localelectrode 402 is set at an attractive potential Vle, with a periodicattractive overvoltage Vo being applied to induce sequential evaporationof ions from the specimen 10 towards a detector 404. The detector 404 isconnected to a positioning stage 406 to allow some degree of adjustmentof its field of view.

A first intermediate electrode 408 is here used as a filteringintermediate electrode which applies a filtering potential (heredesignated Vf), as in the atom probe 300, to reduce the detection ofspurious ions. Whereas the filtering intermediate electrode 306described with respect to version 300 (FIG. 3) of the invention wasdepicted as having a conventional apertured plate-like form, thefiltering intermediate electrode 408 is here shown as having afrustoconical tubular form with an elongated interior passage orientedalong the axis of the (nominal/unaltered) flight cone A. Thus, apossible path for spurious ions rejected by the filtering intermediateelectrode 408 is illustrated at B. An enhancement is provided in theform of a screen 410, which extends across the interior passage of thefiltering intermediate electrode 408 to better distribute its electricfield across the diameter of the flight cone. Such screen preferablytakes the form of an electroformed metal mesh, or other radiatingmembers having a high proportion of free space (typically≧93%) inrelation to the diameter of its constituent members, whereby ion flightpassage is not significantly inhibited.

The filtering potential will also decelerate desired ions, resulting intheir spread as shown at C′. This can be corrected by use of a secondintermediate electrode 412, which is used as a flight modificationintermediate electrode as in the atom probe 200. By applying anappropriate attractive potential Vi at the flight modificationintermediate electrode 412, the desired ions can be reaccelerated withinthe length of the electrode 412 to modify their flight path as shown atC″, ultimately resulting in flight cone C. While the second intermediateelectrode 412 is depicted as having a tubular geometry of uniformdiameter, it too may be formed as a tube having a frustoconical or othershape.

The foregoing uses of the filtering intermediate electrode 408 and/orthe flight modification intermediate electrode 412 allow achievement ofboth of the aforementioned benefits of noise reduction and field of viewmodification. Since modification of the flight cone (and thus the fieldof view) will also depend on the location along the flight cone at whichflight modification occurs, the intermediate electrodes 412 and 408 maybe provided with positioning stages 414 and 416 as well to allow furtherversatility in field of view modification.

The foregoing version 400 is also beneficial in that either or both ofthe intermediate electrodes 408 and 412 may be operated as flightmodification intermediate electrodes, or as filtering intermediateelectrodes, by application of the appropriate constant or periodicpotentials to the electrodes. When the intermediate electrodes 408 and412 are appropriately configured, they may be provided in a telescopingarrangement or other arrangement wherein one of the electrodes is atleast partially situated within the other, thereby addressing spaceissues (particularly where the positioning stage 406 is used) andfurther enhancing the versatility of the filtration/flight modificationeffects that can be achieved by the electrodes.

The various preferred versions of the invention are shown and describedabove to illustrate different possible features of the invention and thevarying ways in which these features may be combined. Apart fromcombining the different features of the foregoing versions in varyingways, other modifications are also considered to be within the scope ofthe invention. Following is an exemplary list of such modifications.

First, while the intermediate electrode 206 of FIG. 2 was characterizedas having a tubular form, this does not require that an intermediateelectrode 206 take the form of a circular tube in order to effectchanges in field of view. Tubes of other shapes—such as conical tubes,or tubes having other variations in their diameters along their length,or even tubes having non-circular diameters (e.g., polygonalcross-sections)—are possible. However, it must be kept in mind that thecomplexity of the electric field emitted by the intermediate electrode206 will generally increase as its shape grows more complex, and thiswill affect the complexity of the data analysis needed to properlyinterpret the data from the detector 202.

Second, while the foregoing versions of the invention illustrate atomprobes with a single flight path modification intermediate electrode, asingle filtering intermediate electrode, or a combination of theforegoing, it should be understood that more than one of either type ofelectrode may be used. In particular, multiple flight path modificationintermediate electrodes arrayed in sequence along a flight path may beuseful.

Third, while the foregoing versions of the invention illustraterepositioning of any one or more of the detector, the flight pathmodification intermediate electrode, and the filtering intermediateelectrode, it is also possible to attain further modification of fieldof view (and of associated properties such as magnification and samplingdepth) by repositioning the specimen along the axis of the flight cone.Repositioning of specimens in planes perpendicular to the axis of theflight cone is known (see, e.g., International Publication WO 99/14793at page 8 lines 16–26); however, as discussed therein (and in U.S. Pat.No. 5,440,124 at column 9 lines 38–44), a more conventional arrangementis to laterally reposition the local electrode, detector, and anyintermediate electrodes with respect to the specimen mount. This isowing to practical difficulties in repositioning the specimen, which isgenerally already repositioned on gimbals (which allow rotation of thespecimen to expose desired faces to the local electrode) and which isalso generally associated with cryogenic cooling equipment. Owing to thebulk of the gimbals and cooling equipment, it is usually difficult topractically reposition the specimen mount in the lateral planes aboutthe axis of the flight cone. However, such an arrangement is achievable,and it is similarly possible to reposition the specimen mount indirections parallel to the flight cone axis as well.

The invention is not intended to be limited to the preferred versions ofthe invention described above, but rather is intended to be limited onlyby the claims set out below. Thus, the invention encompasses alldifferent versions that fall literally or equivalently within the scopeof these claims.

1. An atom probe comprising: a. a specimen mount whereupon a specimen tobe analyzed may be situated; b. a detector spaced from the specimenmount; c. a local electrode situated between the specimen mount anddetector; d. a filtering electrode situated between the local electrodeand the detector; wherein: (1) the local electrode and the detector areeach charged to a potential with respect to the specimen mount wherebyions from any specimen provided on the specimen mount are attractedtowards the local electrode and the detector, and (2) the filteringelectrode bears a filtering potential with respect to the specimenmount, the filtering potential being closer to the potential of thespecimen mount than to the potential of the local electrode.
 2. The atomprobe of claim 1 wherein the filtering potential is at leastsubstantially equal to the potential of the specimen mount.
 3. The atomprobe of claim 1 wherein the filtering potential is intermittentlyapplied to the filtering electrode.
 4. The atom probe of claim 1 whereinthe filtering electrode is tubular, and has an interior length definedtherein oriented along an axis between the specimen mount and thedetector, wherein ions traveling between any specimen on the specimenmount and the detector travel through the interior length of thefiltering electrode.
 5. The atom probe of claim 4 wherein the filteringelectrode includes one or more radiating members extending across itsinterior.
 6. The atom probe of claim 1 further comprising anintermediate electrode situated adjacent the filtering electrode betweenthe local electrode and the detector, wherein the intermediate electrodeis charged to a potential between that of the local electrode and thedetector.
 7. The atom probe of claim 1 further comprising anintermediate electrode situated between the filtering electrode and thedetector, wherein the intermediate electrode is charged to a potentialbetween that of the filtering electrode and the detector.
 8. The atomprobe of claim 7 wherein the intermediate electrode is charged to apotential with respect to the specimen mount which is at least as greatas the potential of the local electrode with respect to the specimenmount.
 9. The atom probe of claim 1 further comprising a tubularintermediate electrode adjacent the filtering electrode, theintermediate electrode having an interior length defined thereinoriented along an axis between the specimen mount and the detector,wherein ions traveling between any specimen on the specimen mount andthe detector travel through the interior length of the intermediateelectrode.
 10. The atom probe of claim 1 wherein at least one of thespecimen mount and the detector are movable to adjust the distancebetween the specimen mount and the detector.
 11. The atom probe of claim10 wherein the filtering electrode is repositionable between thespecimen mount and the detector.
 12. The atom probe of claim 11 furthercomprising an intermediate electrode situated between the localelectrode and the detector.
 13. The atom probe of claim 12 wherein theintermediate electrode is also repositionable between the specimen mountand the detector.
 14. The atom probe of claim 12 wherein one of theintermediate electrode and the filtering electrode is repositionable toat least partially receive the other therein.
 15. The atom probe ofclaim 10 further comprising an intermediate electrode situated betweenthe local electrode and the detector, wherein the intermediate electrodeis repositionable between the specimen mount and the detector.
 16. Theatom probe of claim 15 wherein one of the intermediate electrode and thefiltering electrode is repositionable to at least partially receive theother therein.
 17. An atom probe comprising: a. a specimen mount bearinga datum potential; b. a local electrode spaced from the specimen mount,the local electrode bearing an attractive potential with respect to thedatum potential whereby any ions from a specimen on the specimen mountare attracted toward the local electrode; c. a detector spaced from thelocal electrode and the specimen mount, the detector bearing anattractive potential with respect to the datum potential whereby anyions from a specimen on the specimen mount are attracted toward thedetector; d. a first intermediate electrode situated between the localelectrode and the detector; wherein at least one of the specimen mountand the detector are movable to adjust the distance between the specimenmount and the detector.
 18. The atom probe of claim 17 wherein the firstintermediate electrode bears a potential adjustable about the potentialof the local electrode, whereby the flight path of ions travelingadjacent the first intermediate electrode and between the localelectrode and the detector may be adjusted.
 19. The atom probe of claim17 wherein the first intermediate electrode is a filtering electrodebearing a filtering potential which is: a. between the datum potentialand the potential of the local electrode, and b. closer to the datumpotential than to the potential of the local electrode.
 20. The atomprobe of claim 17 wherein the filtering potential is at leastsubstantially equal to the datum potential.
 21. The atom probe of claim17 wherein the filtering potential is intermittently applied to thefirst intermediate electrode.
 22. The atom probe of claim 17 wherein thefirst intermediate electrode has an interior passage with a lengthextending between the local electrode and the detector.
 23. The atomprobe of claim 22 wherein the intermediate electrode includes one ormore radiating members extending across its interior passage.
 24. Theatom probe of claim 17 further comprising a second intermediateelectrode situated between the local electrode and the detector, whereinthe second intermediate electrode bears a potential between that of thefiltering electrode and the detector.
 25. The atom probe of claim 24wherein one of the first intermediate electrode and the secondintermediate electrode is repositionable to at least partially receivethe other therein.
 26. The atom probe of claim 17 wherein theintermediate electrode is repositionable between the specimen mount andthe detector.
 27. An atom probe comprising: a. a specimen mount bearinga datum potential; b. a detector spaced from the specimen mount; c. alocal electrode between the specimen mount and the detector; d. anintermediate electrode situated between the local electrode and thedetector; e. a filtering electrode situated between the local electrodeand the detector; wherein: (1) the local electrode, intermediateelectrode, and filtering electrode are located along an ion flight pathbetween the specimen mount and the detector; (2) at least one of thespecimen mount, the detector, the local electrode, the intermediateelectrode, and the filtering electrode are movable along the flightpath; (3) the detector, local electrode, and intermediate electrode eachbear an attractive potential with respect to the datum potential,thereby attracting any ions from a specimen on the specimen mount; and(4) the filtering electrode bears a filtering potential closer to thepotential of the specimen mount than to the potential of the localelectrode.
 28. The atom probe of claim 27 wherein one of theintermediate electrode and the filtering electrode is repositionable toat least partially receive the other therein.
 29. The atom probe ofclaim 27 wherein at least one of the intermediate electrode and thefiltering electrode is tubular, and includes an interior passage havinga length oriented along the ion flight path.
 30. The atom probe of claim27 wherein at least one of the intermediate electrode and the filteringelectrode includes: a. an interior passage oriented along the ion flightpath, and b. includes one or more radiating members extending across itsinterior passage.